Analysis of the interaction of planetary plasma and dust with the magnetic field

卡西尼号宇宙飞船记录的最新等离子体数据表明，土星内部磁层中的尘埃和等离子体以比以前认为的更复杂的方式相互作用（Wahlund等人，2009年）。这些数据是在穿越土星E环时获得的，E环是一个由亚微米和微米大小的水冰颗粒（尘埃）形成的纤细尘埃环（见封面图）。这些颗粒是由活跃的冰卫星土卫二喷出的。正在进行的NASA/ESA卡西尼-惠更斯土星使命任务（Spahn et al.，2006 b;施密特等人，2008年）。在轨道上，颗粒在行星引力、太阳辐射和土星系统电磁场的影响下旋转。它们还与土星内磁层的等离子体相互作用。在过去几十年中，普遍接受的图像是共转等离子体对E环颗粒充电（Horányi等人，1992），感受到感应共旋场（Birmingham和Northrop，1979）和土星磁场的影响，而对等离子体流没有明显的反馈。然而，卡西尼朗缪尔探测器推断的等离子体密度（Wahlund等人，2009年）显示了电子和离子（后者大多是正的，主要是单电荷）的密度之间的明显失配，这是当航天器（垂直）穿过E环（图1，上图）时看到的。同时，测得的离子漂移速度（图1，下图）介于开普勒速度（8 km/s）和共转速度（25 km/s）之间（相对于航天器为20 km/s）。一个简单的解释是，大量电子停留在土星E环的尘埃颗粒上，因此它们的电荷不会直接被朗缪尔探测器记录（Wahlund等人，2009年）。反过来，这些颗粒通过库仑阻力减慢了等离子体的流动，这就解释了离子漂移的速度为什么比共转慢。如果这种情况是正确的，那么该区域的相关电流将导致偏离土星磁场，对E环尘埃的动力学，土星等离子体的演化以及土星磁层这一部分的磁场测量的解释产生影响。初步估计表明，大量的小E环颗粒是必要的，以解释所观察到的顺序，下降到几十纳米和一百纳米之间的大小的电荷失配。该估计采用陡峭的粒度分布（施密特等人，2008），以及对于约-3V的E环区域中的给定电势的适当大小依赖的平衡电荷（Kempf等人，2006年）。这是有趣的，因为卡西尼等离子体光谱仪（CAPS）仪器的测量也暗示了直接在土卫二羽流中大量产生这种大小的颗粒（Jones等人，2009年）。因此，两个独立的测量点存在的人口的小E环颗粒。否则，这一范围的颗粒尺寸很难通过卡西尼CDA的现场测量或通过E环的测光来限制，使用成像子系统（ISS）或视觉和红外测绘光谱仪（VIMS）获得的图像。在拟议的项目中，我们计划扩展现有的模型来估计E环尘埃的配置下降到10纳米的晶粒尺寸，并估计尘埃和等离子体的相互作用，以推导出一个模型的等离子体盘中的相关方位电流和土星磁场的扰动。

Newest plasma data recorded by the Cassini spacecraft suggest that dust and plasma in Saturn s inner magnetosphere interact in a more complex manner than previously thought (Wahlund et al., 2009). The data was obtained during crossings of Saturn s E ring, a tenuous dust ring that is formed by submicron and micron sized grains (dust) of water ice (see cover figure). These grains are ejected by the active ice-moon Enceladus. The dust streams emerging from Enceladus were directly measured by the Cosmic Dust Analyzer (CDA) onboard the ongoing NASA/ESA Cassini-Huygens mission to Saturn (Spahn et al., 2006b; Schmidt et al., 2008). In orbit, the grains revolve under the influence of the planet s gravity, solar radiation, and the electromagnetic field in the Saturn system. They also interact with the plasma in Saturn s inner magnetosphere. The commonly accepted picture in the last decades was that corotational plasma charges the E ring grains (Horányi et al., 1992), which feel the effects of the induced corotational field (Birmingham and Northrop, 1979) and Saturn s magnetic field, while there is no significant feedback on the plasma flow. However, the plasma densities inferred by the Cassini Langmuir probe (Wahlund et al., 2009) show a clear misfit between the densities of electrons and ions (the latter are mostly positive and mainly single charged), which is seen when the spacecraft (vertically) crosses the E ring (Fig. 1, upper panel) near its densest part. Simultaneously, the measured ion drift speeds (Fig. 1, lower panel) are intermediate (20km/s, relative to the spacecraft) between Kepler speed (8km/s) and corotation (25km/s). A simple explanation is that a significant amount of the electrons rests on dust grains in Saturn s E ring, so that their charge is not directly registered by the Langmuir probe (Wahlund et al., 2009). In turn, the grains slow down the plasma flow via coulomb drag, offering an explanation for ion drift speeds slower than corotation. If this scenario is correct then the related currents in this region will induce deviations from the Saturnian magnetic field bearing consequences for the dynamics of the E ring dust, the evolution of Saturnian plasma, and for the interpretation of magnetic field measurements in this part of Saturn s magnetosphere. Preliminary estimates show that a large amount of small E ring grains is necessary to explain a charge misfit of the observed order, down to sizes between tens of nanometers and one hundred nanometers. This estimate employs a steep grain size distribution (Schmidt et al., 2008), together with the appropriate size-dependent equilibrium charges for a given potential in the E ring region of about -3V (Kempf et al., 2006). This is interesting, since also measurements by the Cassini Plasma Spectrometer (CAPS) instrument hint at the abundant production of grains of this size directly in the plume of Enceladus (Jones et al., 2009). Thus, two independent measurements point to an existence of a population of small E ring grains. Otherwise, this range of particle sizes is hard to constrain by in situ measurements by Cassini CDA or by photometry of the E ring, using images obtained by the Imaging Subsystem (ISS) or the Visual and Infrared Mapping Spectrometer (VIMS). In the proposed project we plan to extend existing models to estimate the configuration of E ring dust down to grain sizes of 10 nm, and estimate the interaction of dust and plasma, in order to derive a model for the related azimuthal currents in the plasma disk and for the so induced perturbations of the Saturnian magnetic field.